It is known that optical fiber technology has a wide variety of applications in communication systems. Copper lines, coaxial cables, and in certain cases microwave relays and satellites, have been replaced with optical fiber communication systems. Optical fiber systems are particularly attractive with regard to long-distance communication, because it has the ability to transmit enormous information and is less susceptible to electromagnetic interference. Optical fiber links are useful in transmitting a signal over an extremely short distance, for example, between a large general-purpose computer and its peripheral equipment. Optical fiber transmission systems are also employed to transmit a signal between circuit boards within a computer. Various investigations have also been made with respect to the connection of optical fibers at a low level, such as the connection between microchips. Optical fiber systems are also employed as sensors for sensing pressure, liquid level, temperature, magnetic field, acidity, and other stimuli. These optical fiber sensors are generally based on a conversion mechanism that depends on a change in the polarization of light passing through the sensors. The light polarization is changed by an external stimulus.
Optical fiber systems are equipped with three main components, which include a transmitter for converting a data signal to an optical signal, an optical fiber for guiding the optical signal, and a receiver for capturing the optical signal at the other end of the optical fiber and converting it to an electrical signal. A light source in the transmitter can be a semiconductor laser diode or a light-emitting diode (LED). An LED is a light source of relatively low output which operates at a lower data transmission ratio than that of a laser diode. For applications requiring high-speed data transmission or long-distance communication, a laser diode is preferred. Light emitted from a light source is modulated to convert an optical signal to a data signal.
The linearly polarized light produced by the semiconductor laser diode is very effective in many communication or measurement systems employing optical fibers. However, the extinction ratio of light emitted from the laser diode is approximately 20 dB, and this value is not always stable. Also, it cannot be said that the extinction ratio is sufficiently high for many applications. For instance, there is demand for an extinction ratio of 40 dB or greater. To meet this demand for a laser light having such a high extinction ratio, a laser module with a polarized-wave rotating function has been put into practical use as a light source, and in an optical transmission path, a fiber polarizer, etc., have been put into practical use. These elements generally require the following technique and construction. The technique and construction is to pass laser light through a linear polarizer having a higher extinction ratio than that of the laser light being guided. To achieve the technique and construction, a multiplicity of methods have been proposed.
A linear polarizer that is employed for the aforementioned object is used as a single element. A Glan-Thompson prism, a PBS, POLARCOR (trade name), etc., are polarizers that are commonly used. These linear polarizers generally require a lens system to couple with an optical fiber. A lens that is employed in the lens system is generally costly, and it is necessary to align the lens with an optical fiber in three dimensions with submicron accuracy to couple them. This alignment step is extremely complicated and difficult. Therefore, there is demand for a reduction in the number of components and simplification of the alignment step.
If the thickness of a polarizer is so thin that there is no considerable loss, when disposed between optical waveguides, a lens will not be required in order to couple the polarizer with the optical fiber. Furthermore, a certain construction makes it possible to apply a polarizer without a complicated alignment step. As a linear polarizer with such an extremely thin thickness, there is known “LAMIPOL” (trade name) whose thickness is about 30 μm. “LAMIPOL” is employed in optical-fiber applications employing no lens. However, the effective aperture of“LAMIPOL” is so small that the resultant assembly step is fairly difficult. Therefore, the step is still complicated. Since “LAMIPOL” has a laminated structure, the optical characteristics vary largely because of a change in the incidence angle of light. Because of this, the inclination of an element for suppression of reflected return light cannot be utilized. This element inclination is indispensable for optical-fiber applications and puts restrictions on the application of “LAMIPOL.” In addition, since “LAMIPOL” consists of alternate layers of which thermal expansion coefficients are significantly different, there is another problem that the optical characteristic will be lost by high-energy irradiation such as high-power laser irradiation and high-temperature processing. Therefore, “LAMIPOL” is limited to applications where light intensity is relative low, as in fiber polarizers and polarized-light output fiber collimators. Thus, an optical system employing “LAMIPOL” has the advantage of not using a lens, but, as described above, is rather restricted in applicable use.
On the other hand, a wavelength selecting filter, which is employed for wavelength-multiplexing communication employing optical fibers, can be made nearly the same thickness as the above-mentioned “LAMIPOL.” Therefore, the wavelength selecting filter can be applied to optical fibers without using lenses. For example, a slit is formed in a connector so that it crosses an optical fiber, and the wavelength selecting fiber is inserted into the slit. In addition, some wave plates made of polyimide, etc., can be formed to a thickness less than the aforementioned thickness, and there has also been provided an element with a slit into which a wave plate is inserted. These filters require no lens because they are thin. However, since the optical characteristic of the optical filter depends largely on an angle of incidence, an additional design is required for the filter, and the slit needs to be formed with a high degree of accuracy so that the filter can be inserted at a desired angle.